1. Field of the Invention
The present invention relates to harmonic distortion and power factor measurements and, more specifically, to determining the total harmonic distortion and power factor of a non-linear load circuit coupled to an a.c. power source generating a.c. voltage and a.c. current, the a.c. current input to the load circuit being substantially in phase with the a.c. voltage provided across the non-linear load circuit by the a.c. power supply.
2. Description of the Related Art
Power factor is essentially a measurement of the efficiency of the use of a.c. power by a given load. A power factor equal to one (1) means the load is perfectly efficient.
Power factor considerations for electronic and electrical circuits and devices are becoming increasingly important in industry. It is in the best interest of any manufacturer of electronic and electrical devices to produce products which are as efficient as possible. This allows the manufacturer to promote improved performance specifications and to meet increasingly stringent industry standards. One technique used to improve the power factor of non-linear load devices includes adding active power factor correction circuitry to the device. However, even active power factor correction circuits cannot produce perfectly efficient devices.
Present techniques for determining the power factor of a device coupled to an a.c. power source may not provide an accurate power factor measurement, and can actually establish the device to be less efficient than it really is. These techniques assume an ideal a.c. power source where the a.c. voltage and a.c. current are perfect sinusoids, and attribute all inefficiencies or contributions to an observed power factor in the test circuit (consisting of the a.c. power source and the device under test) to the device under test, when in fact some of the circuit inefficiencies or contributions to the observed power factor are caused by the a.c. power source. This unfairly attributes all inefficiencies or contributions to the observed power factor to the device under test, and does not accurately reflect the true power factor of the device under test.
Generally, power factor (PF) is defined as the ratio of real power to apparent power: ##EQU1##
FIG. 1A shows a typical circuit representation of a load circuit under test for power factor and harmonic distortion considerations. Load circuit 5, having an impedance Z.sub.L, is coupled to a.c. power source 6 having an impedance Z.sub.S. The a.c. current i input to the load circuit 5 can be measured at the load using conventional techniques. Similarly, the a.c. voltage v at the input of the load circuit 5 caused by the a.c. power source 6 can be measured using conventional techniques.
If the system is purely resistive, Z.sub.L =R.sub.l and Z.sub.S =R.sub.S then, as shown in FIG. 1B, the voltage v and current i waveforms at the load circuit 5 are sinusoidal and in phase, assuming an ideal power source 6. Further, the power waveform p is the product of v and i at any one instant. This is also shown by phasor a in the phasor diagram of FIG. 1D, which represents the real power at the load circuit.
In practical applications, these ideal circumstances are rarely achieved and inefficiencies or contributions to the observed power factor exist. One contributing factor to these inefficiencies comes from the devices themselves which produce a phase difference between the fundamental components of the a.c. current and a.c. voltage at the load circuit. This is shown in FIG. 1C where the current waveform i lags the voltage waveform v by a phase angle .phi., and in FIG. 1D where phasor b, representing the fundamental apparent power, is offset from the real power phasor by angle .phi..
An additional contributing factor to the inefficiencies or the observed power factor of an electronic or electrical device comes from distortions of the voltage and current at the load circuit. This is shown in FIG. 1D as phasor c which represents to the total apparent power and which is offset from the fundamental apparent power by an additional phase angle .delta..
Thus, as shown by FIG. 1D, the general power factor equation of equation (1) can also be expressed as: EQU PF=cos.phi.cos.phi. (2)
where cos.phi. corresponds to the contributions to apparent power due to phase differences between the fundamental components of the voltage and current at the load, and cos.delta. corresponds to contributions to apparent power due to the harmonic components of voltage and current at the load.
The present invention applies to non-linear load circuits at which the fundamental components of the a.c. voltage and a.c. current are substantially in phase, hence where there is no power factor degradation due to phase angle .phi.. In other words, since phase angle .phi. is substantially equal to zero, cos.phi. is equal to one, and the power factor degradation is attributable only to the harmonic distortions.
While it is known to determine power factor as a function of harmonic distortions at the load circuit, known measurement techniques in considering harmonic distortions typically consider only current distortions and assume that the a.c. voltage waveform is a distortion-free sinusoid and does not contribute harmonic distortions to the power factor determination. For example, typically the total harmonic distortion of a non-linear load circuit is determined by measuring the total harmonic distortion of only the a.c. current (THD.sub.i) at the load circuit.
The power factor of the non-linear load circuit is then determined based on THD.sub.i, according to the relationship: ##EQU2##
Although this technique does provide a harmonic distortion and power factor measurement for non-linear load circuits when the current and voltage waveforms are in phase, it does not accurately reflect the power factor of the load circuit because it does not consider the total harmonic distortion and assumes that the a.c. voltage at the load circuit is a distortion-free sinusoidal waveform. In reality, this assumption is almost never true. A.c. power supplied by a utility company to a building generally encounters an internal non-linear impedance in the building which makes the a.c. voltage waveform output at one of the building's electrical wall sockets to be distorted.
In addition, the voltage waveform applied to the load circuit may vary depending on where the harmonic distortion or power factor measurement is being performed. For example, an a.c. voltage waveform from a utility a.c. power source in one room or on one floor of a building is often different from an a.c. voltage waveform from the same utility a.c. power source in another room or on another floor of the same building. Similarly, the voltage waveforms of the same utility a.c. power source will differ at different buildings.
Moreover, methods and devices for determining harmonic distortions and power factors which require an assumption that the a.c. voltage waveform at the load circuit is a distortion-free sinusoidal waveform provide inaccurate measurements because power factor degradation caused by the non-sinusoidal a.c. voltage of the a.c. power source is attributed to the load circuit, when in fact it comes in part from the a.c. power source connected to the load circuit.
Special purpose instruments exist for determining the power factor of a load circuit using the general expression: ##EQU3## However, these special purpose measurement instruments assume an ideal a.c. power source and have a number of other performance characteristics that limit their flexibility and applications. For example, these instruments often have limited bandwidth which limits the accuracy of power factor determination. They also suffer from lack of resolution when the power factor is very close to 1. Further, V.sub.rms and i.sub.rms measurements attribute the harmonic components of both v and i to the load circuit. This provides an inaccurate power factor measurement of the load circuit because distortions of the a.c. power source supplying the a.c. voltage are incorrectly attributed to the load circuit under test.
Some conventional techniques for determining total harmonic distortions and power factors of non-linear load circuits have attempted to account for the influence of harmonic distortions caused by the a.c. power source, by imposing standards for distortion of the a.c. voltage, when making current distortion measurements. For example, European Standards IEC-555-2 and Military Standards MIL-STD-1399 (Navy) .sctn.300A, list tolerance ranges of total voltage harmonic distortion when measuring the total current harmonic distortion at the load circuit. This fails to account for the actual distortion of the a.c. voltage across the load circuit.